Method of producing biocides from industrial process waters

ABSTRACT

Method of producing biocides from an aqueous flow of process water, said method comprising the step of subjecting a water flow containing ions which give rise to conductivity through an electrolysis cell in order to generate chemicals with biocidal performance. The method can be used for treating fresh and waste water systems, such as water streams of cooling systems, fermentation, mining and biorefining, for example papermaking process waters. It can be used for reducing halogen concentration.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to the production of biocides. In particular the present invention concerns a method of producing biocides from industrial process waters.

2. Description of Related Art

Conditions in a papermaking process are often favorable for microbes to grow [1]. Microbes in the process can cause a multitude of production problems, from decreased production efficiency via impaired runnability and raw material spoilage to product safety issues [2, 3, 4].

To a great extend microbial control in aqueous systems is based on addition of chemicals (biocides) into the process. They act either by killing microorganisms or by inhibiting the growth of micro-organisms. An ideal biocide should meet several requirements such as: applicability over a wide range of operating conditions, no interference with other additives, broad spectrum of activity towards microbes, efficient and fast-acting, environmentally friendly and non-toxic, safe for the operator, low-cost, and easy-to-handle [2]. Unfortunately, there is no biocide that can encompass all the requirements, and none of the biocides is suitable for all applications.

The development of a biocide strategy for a paper mill is always a compromise between the costs and performance. An insufficient use of biocides endangers the machine runnability and product quality [3, 5]. On the other hand, extensive use of biocides is not only expensive, but may result in unwanted interactions with the process and other chemicals [6, 7]. During the past years the biocide development has been rapid. Reductive biocides were first replaced by strong oxidizers. After noticing the problems with the strong oxidizers [7] the development has been towards weak oxidizers and stabilized halogens. Both continuous and batch additions of these biocides have been used [8].

Biocide usage and microbial growth both can cause chemical variations in papermaking processes [9]. Active compounds in predominant biocide programs are salts, they are dosed in certain pH, and they do interact with the process and with other chemicals. Elevated conductivity, charge, and dissolved calcium levels have shown to increase the formation of defects on paper machine [10, 11] have showed that stable chemical conditions together with functioning microbial control enable stable production and acceptable product quality. Elevated and fluctuating conductivity due to the salts added with biocides might be a thread to paper machine runnability. On the other hand, also the problems due to storage and transportation of hazardous materials related to biocide production and use —as well as corrosion and waste water quality issues related to halogen usage, should not be forgotten.

In summary, the technology exhibits several disadvantages. Biocides are hazardous chemicals and therefore the approach involves risks associated with storage and transportation. Long delay between production and dosing expose the biocides to degradation. Especially widely used halogens are associated with corrosion risks. Biocides contain salts which are usually detrimental to the process the chemical are dosed into.

Therefore having a new chemical-free approach to control microbial growth without dosing of these salts would allow economically and environmentally efficient control programs. A technical solution to the indicated disadvantages would not only be economically beneficial but also environmentally sustainable.

SUMMARY OF THE INVENTION

It is an aim of the present invention to eliminate at least a part of the problems of the art and to provide a new way of producing biocides.

The present invention is based on the concept of utilizing industrial process water for producing biocides. Existing technologies do not utilize process waters for biocide generation. In the known technology biocides are generated using brine solutions external to the process. Predominant technologies do not even provide on-site applications.

Inactivation of bacteria in an electrolysis cell has been carried out but the application in industrial process waters has not been conducted.

The present invention provides a method in which an industrial process water flow containing ions causing conductivity is fed through an electrolysis cell. This electrochemical treatment partly converts these compounds into chemicals with biocidal performance. Commercial cells can be used. Cell construction and operation parameters can be modified according to the application.

More specifically, the present invention is mainly characterized by what is stated in the characterizing part of claim 1.

Considerable advantages are obtained by the invention. As the below examples will show in more detail electrolysis, in particular direct electrolysis, of process waters is a promising new technology to control microbial growth in water circulations.

The present technology with (preferably direct) electrolysis of process water is capable of inactivating in practice all commonly present microbes in sample. Thus, the electrolysis disclosed in the examples generated excess amount of biocidal compounds.

The electrolyzed fractions can be utilized as biocide to treat other process flows. The technology has been shown to be effective also in samples with high consistency. The electrolysis performance can be increased by compensating the salt loss with the addition of salt. This technology decreases the conductivity level of the process by decreasing the halogen concentration. This has several advantages from process efficiency, chemical performance, corrosion, and waste water management perspectives.

Based on the above, the present technology finds broad application. Thus, it can be carried out using papermaking process waters (for example shower water, filtrate water, white water, headbox furnish or broke). However, generally, the concept is applicable to any aqueous process requiring microbial control, such as fresh and waste water systems, cooling systems, fermentation, mining and biorefining.

As a practical matter, the electrolysis technology is highly cost-efficient. It does away with the costs of raw materials for producing biocides. It can be estimated that the total costs for applications at, e.g., a paper mill would be on the order of 0.2

/ton of paper.

Next the present technology will be examined in more closely with the aid of a detailed description with reference to the attached drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a bar chart showing the total bacterial count for each sample (in logarithmic scale);

FIG. 2 shows in perspective view an EC-electro MP-cell;

FIG. 3 indicates in graphical form the experimental setup with one compartment MP-cell;

FIG. 4 shows the effects of current and flow rate on total bacterial count in SUPER sample. The flow rate was fixed at 80 mL/min in the current trials (left), while current levels were fixed at 4A and 7A in the flow rate trial (right);

FIG. 5 indicates biocidability of electrolyzed superfiltrate against (a) original superfiltrate, (b) white water, (c) headbox furnish and (d) broke. Total bacterial count was plotted against different dosage levels;

FIG. 6 shows biocidability of supernatant fractions of (a) white water and (b) headbox furnish. Total bacterial count was plotted against different dosage levels;

FIG. 7 shows free available chlorine in the electrolyzed superfiltrate, as a function of time after electrolysis. Measurement with photometer, Dulcotest DTI (Prominent);

FIG. 8 shows the total bacterial count for the superfiltrate mixtures (original superfiltrate +electrolyzed superfiltrate) in two dosage levels (25% and 33%). They were mixed with time delay to check time dependence;

FIG. 9 indicates the pH of the superfiltrate mixtures (original superfiltrate+electrolyzed superfiltrate, cf. FIG. 5 a);

FIG. 10 shows biocidability of electrolyzed superfiltrate against original superfiltrate at the controlled pH level. (a) Total bacterial count was plotted against different dosage levels. (b) pH was kept at 8.3-8.4 in all the testing points;

FIG. 11 indicates the conductivity of the superfiltrate mixtures (original superfiltrate+electrolyzed superfiltrate, cf. FIG. 5 a);

FIG. 12 shows biocidability of electrolyzed superfiltrate against white water at controlled conductivity by adding sodium chloride. (a) Total bacterial count was plotted against different dosage levels. (b) Conductivity was kept at ˜1.05 mS/cm in all the testing points;

FIG. 13 shows biocidability of electrolyzed superfiltrate against white water at controlled conductivity by adding sodium bicarbonate. (a) Total bacterial count was plotted against different dosage levels. (b) Conductivity was kept at ˜1.07 mS/cm in all the testing points;

FIG. 14 indicates biocidability of electrolyzed superfiltrate against white water at controlled sodium carbonate. (a) Total bacterial count was plotted against different Conductivity was kept at ˜1.07 mS/cm in all the testing points; and

FIG. 15 shows voltage in electrolysis at different conductivities (current was constant as 10A). Conductivity was controlled by adding NaCl or Na₂CO₃.

DESCRIPTION OF PREFERRED EMBODIMENTS

As discussed above, electrochemical generation of oxidants has been studied to find new solutions to control microbial contamination in process waters. The present approach has been used for generating the biocides directly from the process without any chemical additions.

In the present context, the technology has been applied into the papermaking process but it is applicable to any aqueous process requiring microbial control.

As the below examples comprising laboratory electrolysis trials indicate direct electrolysis of process waters is an efficient new concept to control microbial contamination at paper mills. Electrolysis considerably reduces the need of halogen containing biocides, thus lessening risk of corrosion.

Instead of increasing halogen concentration like with the conventional stabilized halogen biocide systems, the electrolysis concept is capable of decreasing the concentration of halogens in the process waters. At the same time the conductivity of the process waters decreased indicating process purifying effect in addition to biocidal effects.

The trials with samples from paper machines indicate that the new concept can be applied into several process stages. In addition to killing effect the electrolysis was able to produce excess amount of active halogens which turned contaminated process water into biocide with substantial biocidal effectiveness. Direct electrolysis of process waters enables on-site biocide production which eliminates all transportation costs, risk associated with storage of hazardous chemicals and biocide lost due to degradation. Thus, biocidal effects together with reduction in amount of halogen containing oxidants and reduction in process conductivity make this concept economically attractive and environmentally positive.

In the method of producing biocides from an aqueous flow of process water, a water flow containing ions, such as halogens, which give rise to conductivity are conducted through an electrolysis cell in order to generate chemicals with biocidal performance. The halogens are typically comprised of chlorine or bromine compounds.

The method comprises, in a preferred embodiment, simultaneously decreasing the conductivity level of the process water by decreasing the halogen concentration.

In particular, the process water flow is subjected to direct electrolysis.

Typically, the process water flow is subjected to electrolysis in order to reduce conductivity of the water with at least 5%, in particular at least 10% and preferably with at least 15 to 85%, e.g. with at least 20%.

In one embodiment, the water is subjected to electrolysis in an electrochemical cell.

Generally, in the present technology, the water is subjected to electrolysis using a current in the range of 0.1 to 1000 A, for example about 1 to 150 A, for example 1 to 100 A. The voltage of the electrolysis varies broadly, from for example about 0.1 to 1000 V, for example the voltage is about 1 to 250 V.

The electrolysis can be carried out for clear water streams. The method can also be carried out for process waters having a consistency of about 0.1 to 20% by mass.

EXPERIMENTAL Sample Materials

Process water and furnishes were taken from several parts of a paper machine at a Finnish fine paper mill (Table 1). Total bacterial count at sampling is shown in FIG. 1.

TABLE 1 Process water and furnishes from a Finnish fine paper mill Sampling point Code Shower water SHOWER Superclear filtrate water SUPER White water WHITE Headbox furnish HEAD Broke BROKE

FIG. 1 shows the total bacterial count for each sample (in logarithmic scale).

Electrolysis Device

The electrochemical cell EC-Electro MP (Electrocell, Denmark) was employed for electrolysis. This is a modular multipurpose cell intended for process evaluations and experimental tests on laboratory scale. The structure of this filter-press type cell is shown in FIG. 2. The projected electrode area was 200 cm2, and the distance between cathode and anode was 3 mm. Titanium was employed as cathode, while DSA (Dimensionally Stable Anode) as anode. DSA is iridium and ruthenium oxide coated titanium. According to the distributor, the ratio of iridium oxide and ruthenium oxide is 70/30. It has high oxygen over-potential and is corrosion-resistant. As a power source, Switch-Kraft Type SK 075B (Kraftelektronik AB, Sweden) was used in the electrolysis. The maximum current and voltage of this rectifier are 50 A and 15V, respectively. All the electrolysis experiments were in the controlled-current mode and anode was continuously cooled down by water circulation system at 5° C. Experimental setup is shown in FIG. 3. Superfiltrate sample was pumped into the cell where electrolysis was taking place. The temperature of the product was monitored after the cell, and the mixture of chlorine and oxygen gases was diluted with air and discharged. pH, ORP (oxidation reduction potential) and conductivity were also measured. All of this data was recorded in a computer.

Detection of Microbes

Treated white water samples were taken aseptically from test trials and transported in sterile plastic vials to laboratory. Samples were cultured within three hours. Logarithmic dilution series were prepared using sterile Ringer's solution. Culturing was performed by pipetting and spreading 1 ml diluted sample on Aerobic Count Petrifilm (AC). Incubation took place in 30° C. for 3 days. Red colonies were counted from AC Petrifilms containing 3 to 300 colonies.

The biomass of samples were studied using ATP biomass kit HS (BioThema, Sweden). Tests were performed according to manufacturer's instructions: 0.05 ml undiluted sample was pipetted in cuvette with reagents, light output was measured and ATP-standard was added and light output was measured again.

Other Analysis

Free available chlorine in the electrolyzed superfiltrate was measured by photometer, Dulcotest DT1 (Prominent, Germany). pH and conductivity were measured using YSI 556 MPS (YSI Incorporated, USA) multi-parameter probe. YSI Professinoal Plus (YSI Incorporated, USA) multi-parameter probe was used to analyse the chloride content.

Results Inactivation of Microbes in Papermaking by Electrolysis

Total bacterial count in the electrolyzed superfiltrate water was given in FIG. 4. Bacteria were killed in electrolysis cell. The number depends on the current and flow rate, i.e. higher current or lower flow rate was more effective to reduce the bacteria. For this filtrate water, >4 A current with <80 mL/min flow rate turned out to be enough to kill almost all bacteria. In practice this indicates, that the approach is more than capable of treating the process samples to ensure microbiologically clean process. In fact, it is probable that in case of high current and/or low flow rate the electrolysis generates excess amount of biocidal compounds.

FIG. 4 shows the effects of current and flow rate on total bacterial count in SUPER sample. Flow rate was fixed at 80 mL/min in the current trials (left), while current levels were fixed at 4 A and 7 A in the flow rate trial (right).

Electrochemically Treated Process Waters as Biocide

As shown in FIG. 1, total bacterial count of superfiltrate was not so high compared with white water, headbox furnish or broke. Therefore one can expect that superfiltrate processed with higher current should still retain biocidability. This was simply checked by mixing the electrolyzed superfiltrate (at 7 A, 80 mL/min) with the original superfiltrate. As shown in FIG. 5 a, it behaved as biocide as expected. 17% dosage was enough to kill 99% of bacteria in the original superfiltrate. It was also mixed with other samples to find its biocidability too (FIGS. 5 b, 5 c, 5 d). Here higher dosage (33%) was required to kill 99% bacteria, simply due to larger number of bacteria in those samples. Also higher fiber consistency of the other samples may have affected the reduced biocide performance. This is well known feature of some oxidants, which are not highly selective in killing but are consumed by all organic material in the sample.

FIG. 5 shows biocidability of electrolyzed superfiltrate against (a) original superfiltrate, (b) white water, (c) headbox furnish and (d) broke. Total bacterial count was plotted against different dosage levels.

In addition to superfiltrate, white water and headbox furnish were also applied for electrolyses. Direct electrolysis was difficult due to their containing solids, thus we simply took the supernatant fraction after sedimentation and applied them for the electrolysis. The processed fractions were then returned back to mix with the original samples at different dosage levels. Supernatant volume was 75% for white water and 35% for headbox, which limited the dosage level. As shown in the FIG. 6, both turned out to function as biocide as well as superfiltrate. As expected, the result of white water supernatant fraction (FIG. 6 a) was similar to that of superfiltrate (FIG. 5 b).

Factors Affecting Biocide Performance of the Electrolysed Process Waters Incubation Time:

Free available chlorine in the electrolyzed superfiltrate was measured by photometer, Dulcotest DTI (Prominent). Measurements were repeated periodically to see the time dependence. As shown in FIG. 7, it decayed with time. Almost all free chlorine disappeared within 3 hours. The biocidability was also decayed accordingly with time (FIG. 8). Mixing right after the electrolysis was the most effective. However, fair effects remained for rather long time.

pH Level:

Generally pH increased with electrolyses. Due to the complex composition of the sample, an unambiguous reason to this cannot be given. Most likely the pH increase is due to formation of alkaline compounds such as NaOH and H2O2. One example is shown in FIG. 9 for superfiltrate, i.e. from pH8 (before) to pH9 (after). Here one may suspect that the biocidability derives simply from increasing pH. Thus we carried out the verification trials with controlled pH. Proper amount of hydrochloric acid (HCl) was added before the electrolysis to compensate in advance. Then the results still showed biocidability in sufficient level (FIG. 10 a). The impact as biocide was somewhat milder than the case without pH control (FIG. 5 a). This could be explained by pH, of course. However, one can also speculate its reason as the reduction of total chlorine amount due to HCl (causing C12 generation). On the other hand, hypochloric acid is known to be the most effective biocidal compound in hypochlorite solution and its concentration is known to increase along decreasing pH. Thus, due to these cross-effects, the effect of pH can be considered as rather insignificant for the biocide performance. For chemical stability of papermaking pH stability is known to be highly important [12, 13]. Therefore pH control of electrolysis flow according to process pH is recommended.

Conductivity

In papermaking free anions and cations have significant roles. The functioning of most of the wet end chemicals is based, at least partly, on charge (e.g. retention aids, fixatives, starch). Charge and conductivity are coupled, and thus any changes in conductivity may cause problems. Naturally, electrolysis gave significant influence on conductivity. One example is shown in FIG. 11 for superfiltrate, i.e. from 1.13 mS/cm (before) to 0.88 mS/cm (after). Drop of more than 20% in conductivity is significant. Current state-of-the-art commercial oxidative biocide systems are mostly based on hypochlorite chemistry. For instance all hydantoin and ammoniumbromine technologies utilize sodium hypochlorite as a halogen source. Commercial hypochlorite generation generates equal amount of salt (C12(g)+2NaOH NaOCl+NaCl+H2O). This salt amount in hypochlorite solution also further increases when hypochlorite decomposes. This means that in practice approximately ⅔ of the hypochlorite is actually sodium chloride (salt).

This salt addition has several disadvantages: Conductivity increase affect chemical interactions of particles in the process causing problems with retention, flocculation etc. Unnecessarily added chloride increases risk of corrosion. Any halogen addition increases the AOX (Adsobable Organic Halogen) load to waste waters. The electrolysis approach eliminates all these disadvantages. No salt is added actually the salt amount is reduced as shown in FIG. 11. This approach actually also enables addition of biocides without detrimental effects.

Thus we carried out the trials with compensating such conductivity drops by salt addition. Here three salts were compared, i.e. sodium chloride (NaCl), sodium bicarbonate (NaHCO3) and sodium carbonate (Na2CO3). Electrolyzed superfiltrate was applied to white water as biocide. Results are shown in FIGS. 12-14.

Among the three, NaCl was the most effective. In terms of energy consumption, NaCl was also effective, i.e. voltage reduction by 1V (8.7V 7.7V at 7A). On the other hand, NaHCO3 and Na2CO3 influenced little on the energy consumption. Extensive addition cases are compared in Fig.15 for NaCl and Na2CO3. The voltage decreased linearly with conductivity increase in both cases.

The addition of salt to the process might lead to increased agglomeration or to problems with retention [14]. Changes in conductivity must be taken into account when selecting the chemicals for optimal process.

Practical Example

Biocide treatment of all loop waters to disc filter of a typical paper machine with production 300.000 t/a, and degree of closure 7.2 m³/t:

In production, cell the electrode area is 16 m2 [15]. Based on this study the capacity of such cell is to produce approximately 0.5 m3/min purified, microbiologically clean process water. The production of the PM is approximately 0.5 t of paper per minute. Process water flow to disc filter is thus approximately 19 m3/min. Therefore the electrolysis is able to treat 2-3% of the process water flow to disc filter. For a typical biocide program this amount should be 5-10%.

As the above examples show, the positive features of the electrolysis technology for the process are obvious: increased process stability, together with the compensation of the disadvantages (due to process closure and accumulation of dissolved and colloidal material).

At the same time, however, the decreased amount of halogens in the process has also positive impacts on corrosion risks and waste water problems. The other positive factors are mostly related to logistics and costs.

The present novel technology does not require any transportation or production of hazardous materials. No biocides need to be transported to the production units. Actually the present technology does not require any transportation at all. Also the raw material for the on-site biocide production is extracted from the process. In case the biocide generation is boosted by the salt addition, only shipping of salt is required. Otherwise only electricity is needed. Also storage needs are minimal since the production can be performed according to the need. This is also recommended due to degradation of active compounds.

REFERENCES

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3. Ludensky, M. (2003). Control and monitoring of biofilms in industrial applications, Int. Biodeter. Biodegr. 51(4), 255-263.

4. Väisänen, O. M., Weber, A., Bennasar, A., Rainey, F. A., Busse, H.-J. and Salkinoja-Salonen, M. S. (1998). Microbial communities of printing paper machines. J. Appl. Microbiol. 84, 1069-1084.

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8. Schrijver, J., and Wirth, B. (2007). Biocides for deposit control in the production of corrugated base paper. Prof. Pap. 2/2007, 37-43.

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1. Method of producing biocides from an aqueous flow of process water, said method comprising the step of conducting a water flow exhibiting a property of conductivity through at least one electrolysis cell in order to generate chemicals with biocidal performance.
 2. The method according to claim 1, wherein the process water flow is subjected to direct electrolysis.
 3. The method according to claim 1, wherein the process water flow is subjected to electrolysis in order to reduce conductivity of the water by at least 5%.
 4. The method according to claim 1, wherein the process water flow is selected from the group of fresh and waste water systems.
 5. The method according to claim 1, comprising decreasing the conductivity level of the process by decreasing the halogen concentration.
 6. The method according to claim 1, comprising subjecting the water to electrolysis in an electrochemical cell.
 7. The method according to claim 1, comprising subjecting the water to electrolysis at a current in the range of 1 to 150 A and a voltage of 1 to 250 V.
 8. The method according to claim 1, comprising subjecting the process water having a consistency of 0.1 to 20% by mass to electrolysis.
 9. The method according to claim 1, comprising subjecting the process water to electrolysis in a plurality of electrolysis cells arranged in parallel or serial arrangement.
 10. (canceled) 